Wafer processing method

ABSTRACT

A wafer processing method includes the following steps: forming, on a back side of a wafer including a device layer, a mask to be used in forming grooves in a substrate along streets from the back side of the wafer; applying plasma etching from the back side of the wafer through the mask to form the grooves in the substrate along the streets and to define chip regions surrounded by the grooves; immersing the wafer in water, to which ultrasonic vibrations are being applied, after the etching step, whereby the device layer is cracked or ruptured along outer peripheral edges of the chip regions; and bonding a tape to a front side of the wafer before performance of the water immersion step.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of processing a waferincluding a substrate and a device layer stacked over a surface of thesubstrate and configuring devices. The devices are formed in respectiveregions divided in the device layer by a plurality of intersectingstreets.

Description of the Related Art

So-called plasma dicing has been conventionally employed to divide awafer as a workpiece by plasma etching to increase the yield of chipsper wafer by narrowing widths to be set as streets (intended divisionlines) on a surface of the wafer and also to shorten the time requiredfor the processing.

In a wafer with devices formed thereon, on the other hand, a devicelayer formed of a circuit layer, which configures the devices, and aninsulating layer also exists on streets, and with etching gas suited foretching silicon (a substrate), the wafer therefore involves a problemthat etching of the device layer on the streets is very difficult. Witha wafer having a test element group (TEG) formed primarily of a metalsuch as aluminum or copper on streets, there is a similar problem.

In Japanese Patent Laid-open No. 2016-207737, a method is disclosed as asolution for this problem. According to this method, a water-solubleresin layer is formed on a front side of a wafer, and after removal ofthe water-soluble resin layer on streets together with a device layerand a TEG by irradiation of a laser beam, plasma dicing is performedfrom the front side of the wafer.

SUMMARY OF THE INVENTION

Any attempt to completely remove a TEG, a device layer and the like byirradiation of a laser beam, however, requires significant power,leading to another problem of formation of affected regions in asubstrate by the irradiation of the laser beam, and hence a reduction inflexural strength of device chips to be formed by division.

Further, if plasma dicing is performed from the front side of a wafer,there is a potential problem that, unless a mask is formed with auniform thickness to protect devices, the mask may be removed at thinmask areas during plasma etching, resulting in exposure of the frontside of the wafer and damage to the corresponding devices.

The present invention, therefore, has as an object thereof the provisionof a method of processing a wafer, which upon dividing the wafer intochips, avoids a reduction in the flexural strength of the chips byirradiation of a laser beam and also avoids damage to devices in plasmadicing.

In accordance with an aspect of the present invention there is provideda wafer processing method including a substrate and a device layerstacked over a surface of the substrate, the wafer having devices formedin respective regions divided by a plurality of intersecting streets.The wafer processing method includes a mask forming step of forming, ona back side of the wafer, a mask to be used in forming a plurality ofetched grooves in the substrate along the streets from the back side ofthe wafer, a plasma etching step of applying plasma etching from theback side of the wafer through the mask after performing the maskforming step, thereby forming the etched grooves in the substrate alongthe streets and defining chip regions surrounded by the etched grooves,a water immersion step of immersing the wafer in water, to whichultrasonic vibrations are being applied by an ultrasonic vibrator, afterperforming the plasma etching step, thereby forming cracks in the devicelayer along outer peripheral edges of the chip regions or rupturing thedevice layer along the outer peripheral edges of the chip regions, and atape bonding step of bonding a tape to a front side of the wafer beforeperforming at least the water immersion step. The devices are separatedfrom the tape after the performing the water immersion step.

Preferably, the wafer processing method of the present invention furtherincludes, before performing at least the water immersion step and thetape bonding step, a guide groove forming step of forming guide groovesin the device layer along the streets from the front side of the waferby a cutting blade or a laser beam without allowing the guide grooves toreach the substrate.

Preferably, in the water immersion step, the wafer is immersed in adirection such that the ultrasonic vibrator and the back side of thewafer face each other.

The wafer processing method according to the present invention performsthe mask forming step of forming, on the back side of the wafer, themask to be used in forming etched grooves in the substrate along thestreets from the back side of the wafer and the plasma etching step ofapplying plasma etching from the back side of the wafer rather than thefront side of the wafer through the mask after the performing the maskforming step to form the etched grooves in the substrate along thestreets, thereby avoiding the occurrence of a situation such that duringthe etching, the front side of the wafer is exposed at areas where themask is thin, and the corresponding devices are damaged. Afterperforming the plasma etching step, the wafer processing methodaccording to the present invention also performs the water immersionstep of immersing the wafer in water, to which ultrasonic vibrations arebeing applied by the ultrasonic vibrator, after performing the plasmaetching step, thereby forming cracks in the device layer, which has notbeen processed by the plasma etching, along outer peripheral edges ofthe chip regions or rupturing the device layer along the outerperipheral edges of the chip regions, and the tape bonding step ofbonding the tape to the front side of the wafer before performing atleast the water immersion step. After performing the water immersionstep, the devices are separated from the tape (for example, by pickingup the device chips), whereby the device chips can be obtained. Thewafer processing method according to the present invention, therefore,does not involve a problem of causing a reduction in the flexuralstrength of chips by the irradiation of a laser beam of high power.

The wafer processing method according to the present invention mayfurther include, before the performing at least the water immersion stepand tape bonding step, the guide groove forming step of forming guidegrooves in the device layer along the streets from the front side of thewafer by the cutting blade or the laser beam without allowing the guidegrooves to reach the substrate, so that with the guide grooves servingas starting points in the water immersion step, cracks are formed in thedevice layer or the device layer is ruptured. It is, therefore, possibleto avoid damage to the devices and also to prevent the device layer fromhaving dimensions greater than the regions (chip regions) surrounded bythe etched grooves (protruding parts of the device layer from havinglarger dimensions).

In the water immersion step, the wafer is immersed in a direction sothat the ultrasonic vibrator and the back side of the wafer face eachother. This mode can facilitate to form cracks in the device layer ofthe wafer immersed in the water, to which ultrasonic vibrations arebeing applied, along the outer peripheral edges of the chip regions orto have the device layer ruptured along the outer peripheral edges ofthe chip regions.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view depicting an example of a wafer;

FIG. 2 is a fragmentary cross-sectional view depicting an example of thewafer;

FIG. 3 is a cross-sectional view illustrating a state in which guidegrooves are formed along streets from a front side of the wafer byirradiating a laser beam without allowing the guide grooves to reach asubstrate;

FIG. 4 is a fragmentary cross-sectional view depicting, on an enlargedscale, the guide grooves formed in the wafer without reaching thesubstrate;

FIG. 5 is a perspective view illustrating the wafer supported by anannular frame with a tape bonded to the front side of the wafer;

FIG. 6 is a cross-sectional view illustrating a state in which awater-soluble resin layer is formed on a back side of the wafer by usinga spin coater;

FIG. 7 is a cross-sectional view depicting, on an enlarged scale, aportion of the wafer with the water-soluble resin layer formed on theback side of the wafer;

FIG. 8 is a cross-sectional view illustrating a state in which a mask isformed on the back side of the wafer by irradiating a laser beam alongthe streets onto the water-soluble resin layer on the wafer and removingthe water-soluble resin layer;

FIG. 9 is a cross-sectional view illustrating, on an enlarged scale, aportion of the wafer with the mask formed on the back side thereof;

FIG. 10 is a cross-sectional view depicting an example of a plasmaetching apparatus that applies plasma etching to the wafer;

FIG. 11 is a cross-sectional view depicting a portion of the wafer towhich plasma etching has been applied;

FIG. 12 is a cross-sectional view depicting an example of a water bathincluding an ultrasonic vibrator;

FIG. 13 is a cross-sectional view illustrating a state in which thewafer is immersed in water, to which ultrasonic vibrations are beingapplied by the ultrasonic vibrator, and cracks are formed in the devicelayer along outer peripheral edges of chip regions or the device layeris ruptured along the outer peripheral edges of the chip regions; and

FIG. 14 is a cross-sectional view illustrating a state in which a deviceis being separated from a tape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A description will hereinafter be made about the individual steps of thewafer processing method according to the present invention when theprocessing method is performed to divide a wafer W depicted in FIG. 1into chips including devices D.

The wafer W depicted in FIGS. 1 and 2 is a circular semiconductor waferincluding a substrate W1 made, for example, from silicon. As depicted inFIG. 2, a device layer D1 is stacked over a front side W1 a of thesubstrate W1, and a plurality of devices D is formed in a matrix patternin the device layer D1. The device layer D1 is configured of a circuitlayer formed from a metal, and an insulating layer (for example, a low-klayer) insulating between circuits. The individual devices D are dividedby a plurality of streets S set on the front side W1 a of the substrateW1 so that the streets S intersect one another at right angles. A backside Wb of the wafer W (a back side of the substrate W1), the back sideWb being a side opposite to the front side Wa of the wafer W, the frontside Wa being in turn a front side of the device layer D1, may beprotected, for example, by an undepicted protection tape bonded thereto.

For example, at least one TEG, which is not depicted and is configuredwith a predetermined metal, may be formed on at least one of the streetsS. Further, undepicted guard rings may be arranged along outerperipheral edges of the devices D, respectively. The guard rings areregions, each of which is configured of a metal and has a width ofseveral micrometers or so, and serve to prevent chipping or cracks fromreaching the devices D upon rupture or the like of the device layer D1.The substrate W1 may also be configured, in addition to silicon, withgallium arsenide, sapphire, gallium nitride, silicon carbide or thelike.

(1) Guide Groove Forming Step

In this embodiment, guide grooves are first formed in the device layer Dalong the streets S from the front side Wa of the wafer W, for example,by a laser beam without allowing the guide grooves to reach thesubstrate W1

For example, the wafer W is carried to a laser processing apparatus 1illustrated in FIG. 3. The laser processing apparatus 1 includes atleast a chuck table 10 configured to hold the wafer W under suction, andlaser beam irradiation means 11 configured to enable irradiation of alaser beam of a wavelength having absorption in the device layer D1 ofthe wafer W held on the chuck table 10.

The chuck table 10 is rotatable about an axis that extends in adirection of a Z-axis, and moreover is reciprocatable in a direction ofan X-axis as a processing feed direction and in a direction of a Y-axisas an indexing feed direction, the Y-axis intersecting the X-axis atright angles, by unillustrated moving means. The chuck table 10 includesa flat holding surface 10 a, which has, for example, a circular shape asits outer shape, is formed of a porous member, and sucks the wafer W.Under a suction force generated by an unillustrated suction source,which communicates to the holding surface 10 a, and transmitted to theholding surface 10 a, the chuck table 10 can hold the wafer W on theholding surface 10 a under suction.

The laser beam irradiation means 11 allows the laser beam, which hasbeen emitted from the laser oscillator 119, to enter a condenser lens111 a inside a condenser 111 via a transmission optical system, wherebythe laser beam can be precisely focused and irradiated onto the devicelayer D1 of the wafer W held on the chuck table 10. The height positionof a focal point of the laser beam is configured to be adjustable in adirection of a Z-axis by unillustrated focal point position adjustingmeans.

The laser processing apparatus 1 includes unillustrated control meansconfigured to perform control of the entire apparatus. By the controlmeans configured of a central processing unit (CPU) and storage devicessuch as memories, moving operations of the chuck table 10 in thedirections of the X-axis and Y-axis, the average output of the laserbeam to be irradiated from the laser beam irradiation means 11, and thelike are controlled.

In the laser processing apparatus 1, the wafer W is held on the holdingsurface 10 a of the chuck table 10 under suction with the front side Wadirected upward. The positions of the streets S, which are to serve asreferences for irradiating the laser beam onto the device layer D1, arethen detected by alignment means 15 illustrated in FIG. 3. Describedspecifically, an image of the streets S on the front side Wa of thewafer W is captured by image pickup means 150 such as, for example, acamera, and on the basis of the captured image so formed, the alignmentmeans 15 performs image processing such as pattern matching and detectsthe coordinate positions of the streets S on the wafer W.

Concomitantly with an input of the detection results of the positions ofthe streets S to the unillustrated control means, the control meanscontrols the moving direction and moving amount of the chuck table 10.Described specifically, the chuck table 10 is indexed and fed in thedirection of the Y-axis to perform an alignment in the direction of theY-axis between desired one of the streets S, onto which the laser beamis to be irradiated, and the condenser 111. This alignment is performed,for example, so that the center line of the desired one street S islocated right below the focal point of the condenser 111.

After the performance of the alignment, the chuck table 10 is furthermoved to a position slightly offset in the direction of the Y-axis,whereby the position right below the focal point of the condenser 111slightly deviates from the position of the center line of the desiredone street S to a position that is apart from the center line by thedistance of the offset in the direction of the Y-axis.

The distance of the offset has been preset in view of the value of thewidth of each street S or the like, and has been stored beforehand inthe unillustrated control means. By setting the distance of the offsetwhile further considering, in addition to information on the width ofeach street S, information (theoretical values and experimental values)on distances between individual masks to be formed in a below-describedmask forming step and information (theoretical values and experimentalvalues) on widths of etched grooves to be formed in a below-describedplasma etching step, two guide grooves M (see FIG. 4) can be formed onthe device layer D1 without any substantial deviation from an intendedposition for the formation of an etched groove indicated by two brokenlines.

Further, the height position of the focal point of a laser beam to befocused by the condenser lens 111 a is aligned, for example, slightlylower than the height position of an upper surface of the device layerD1. The laser oscillator 119 then emits a laser beam of a wavelengthhaving absorption in the device layer D1 so that the laser beam isfocused and irradiated onto the device layer D1. The average output ofthe laser beam is set, for example, low to prevent the resulting guidegroove from reaching the substrate W1.

In addition, the wafer W is fed at a predetermined processing feed ratein a direction of −X (to a side below the plane of the drawing sheet),that is, in a direction out of the drawing sheet. The laser beam isprogressively irradiated onto the device layer D1 along the street S,whereby the device layer D1 is subjected to abrasion and a guide grooveM is formed in the device layer D1 along the street S without reachingthe substrate W1.

When the wafer W has advanced in the direction of −X to a predeterminedposition where the irradiation of the laser beam along the street S isto be ended, the irradiation of the laser beam is stopped. Further, thechuck table 10 is moved in the direction of the Y-axis toward the centerline of the street S, for example, by a distance twice as much as thedistance of the offset. As a consequence, the position right below thefocal point of the condenser 111 is located at a position symmetrical tothe position of the guide groove M formed before in the direction of theY-axis while using the center line of the street S as a reference, inother words, at a position apart from the center line of the street S bythe distance of the offset in the direction of the Y-axis.

The wafer W is fed for processing in a direction of +X (to a side abovethe plane of the drawing sheet), that is, in a direction into thedrawing sheet. Similar to the irradiation of the laser beam in thedirection out of the drawing sheet, the device layer D1 is subjected toabrasion and another guide groove M is formed in the device layer D1along the street S without reaching the substrate W1. In thisembodiment, the two guide grooves M are formed for the single street S,symmetrically with the center line of the street S interposedtherebetween, in the device layer D1. After similar laser beamirradiation has been performed sequentially along all streets Sextending in a first direction, the chuck table 10 is rotated 90 degreesand a similar laser beam irradiation is performed along streets Sextending in a second direction that intersects the first direction atright angles. Therefore, two guide grooves M are formed corresponding toeach of the streets S extending in the second direction on the frontside W1 of the substrate W1 symmetrically with the center line of thecorresponding street S interposed therebetween, in the device layer D1

The average output of the laser beam is set low, and the guide grooves Mare formed without reaching the substrate W1 Therefore, no reduction orthe like occurs in the flexural strength of device chips to beeventually formed by this processing method.

The guide groove forming step may preferably form two guide grooves Mcorresponding to each street S in the device layer D1 as in thisembodiment. As an alternative example, however, a single guide groove Mmay be formed corresponding to each street S, which extends on thecenter line of the street S, in the device layer D1.

In this embodiment, two guide grooves M are formed corresponding to eachstreet S in the device layer D1 by processing feeding of the chuck table10 in the direction out of the drawing sheet and the direction into thedrawing sheet. Two guide grooves M may, however, be formed at the sametime corresponding to each street S in the device layer D1 by processingfeeding of the chuck table 10 in the direction out of the drawing sheet(or in the direction into the drawing sheet). For example, a laser beamemitted from the laser oscillator 119 is allowed to enter the condenserlens 111 a after its splitting into two beam paths by splitting meansconfigured of a half-wave plate, a polarizing beam splitter, a mirrorand the like. The laser beams entered along the two beam paths are thenfocused and irradiated onto the device layer D1.

The guide groove forming step may also be performed using a cuttingapparatus instead of irradiation of a laser beam as in this embodiment.In this case, two guide grooves are formed without reaching thesubstrate W1 by causing a rotating, ultra thin cutting blade to cut intothe device layer D1 along the corresponding street S from the front sideWa of the wafer W.

(2) Tape Bonding Step

To the front side Wa of the wafer W with the guide grooves M formedtherein, a tape T is bonded, for example, as illustrated in FIG. 5. Thetape T is a circular tape having a larger diameter than the wafer W, andis formed of a base material Td, which is made, for example, of apolyolefin resin or the like, and a glue layer Tc on the base materialTd (see FIG. 6). As the glue layer Tc, it is preferred to use anultraviolet (UV)-curable glue that is cured by ultraviolet irradiationto have a reduced adhesive force.

For example, an annular frame F is positioned relative to the wafer W sothat the center of the wafer W mounted on an unillustrated bonding tableand the center of an opening of the annular frame F substantially alignwith each other. The glue layer Tc of the tape T is pressed against andbonded to the front side Wa of the wafer W by a press roller or the likeon the bonding table. By also bonding the glue layer Tc at an outercircumferential part thereof to the annular frame F at the same time,the wafer W is supported on the annular frame F via the tape T so thatthe wafer W can be handled via the annular frame F. As an alternative,the tape T may be bonded to the annular frame F by appropriatelypositioning the wafer W relative to the annular frame F after firstbonding the tape T to the wafer W alone by a press roller or the like.

As the annular frame F and tape T, it is preferred, for example, to usethose having durability to etching gas (for example, SF₆ gas or C₄F₈gas) to be used in the subsequent plasma etching step. As specificexamples, the annular frame F may preferably be formed of stainlesssteel, and the tape T may preferably be formed of a polyolefin or thelike.

(3-1) Formation of Water-Soluble Resin Layer in Mask Forming Step

A mask, which is to be used for the formation of etched grooves in thesubstrate W1 along the streets S from the back side Wb of the wafer W,is then formed on the back side Wb. In this embodiment, a water-solubleresin layer is first formed on the back side Wb of the wafer W.

The wafer W, which has now become ready for handling via the annularframe F, is transferred, for example, to a spin coater 4 depicted inFIG. 6. The spin coater 4 includes, for example, a holding table 40configured to hold the wafer W, rotating means 42 configured to rotatethe holding table 40, and a bottomed cylindrical casing 44 having anopening on an upper end side thereof and accommodating the holding table40 therein.

The holding table 40 has, for example, a circular shape as its outershape, is formed of a porous member or the like, and includes a holdingsurface 40 a communicating to an unillustrated suction source. On acircumference of the holding table 40, fixing clamps 401 are evenlydisposed at predetermined intervals in a circumferential direction tofix the annular frame F. Upon mounting the wafer W, the holding table 40is lifted and positioned at a loading/unloading height position. Uponcoating the suction-held wafer W with the water-soluble resin in aliquid form, the holding table 40 is lowered to a coating heightposition in the casing 44. The holding table 40 is also configured to berotatable by the rotating means 42, which is disposed below, about anaxis that extends in a direction of a Z-axis.

The casing 44 is configured from an outer side wall 440 surrounding theholding table 40, a bottom plate 441 connected to a lower part of theouter side wall 440 and centrally defining an opening in which arotating shaft of the rotating means 42 is inserted, and an inner sidewall 442 arranged upright from an inner circumferential edge of theopening of the bottom plate 441. A cover 444 is fitted on the rotatingshaft, and is disposed between a lower surface of the holding table 40and an upper end surface of the inner side wall 442 of the casing 44 toprevent intrusion of foreign matter into a clearance between therotating shaft and the opening of the bottom plate 441.

In the casing 44, a nozzle 45 is disposed to drop the water-solubleresin in the liquid form onto the wafer W held on the holding surface 40a. The nozzle 45 is arranged extending upright from the bottom plate441, has a substantially L-shaped outer form as seen in a side view, andis turnable about an axis extending in the direction of the Z-axis. Asupply orifice 450, which is formed through a tip portion of the nozzle45, opens toward the holding surface 40 a of the holding table 40.

The nozzle 45 communicates to a water-soluble resin supply source 47, inwhich the water-soluble resin is stored in the liquid form, via a piping47 a and an unillustrated rotary joint. The water-soluble resin storedin the water-soluble resin supply source 47 is, for example,polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) or the like, and isspecifically HogoMax manufactured by DISCO corporation.

The wafer W is mounted on the holding surface 40 a of the holding table40 via the tape T, and is held under suction by the holding table 40.Further, the annular frame F is fixed by the individual fixing clamps401. The holding table 40 with the wafer W held thereon is then loweredto the coating height position in the casing 44. The nozzle 45 is thenturned to position the supply orifice 450 centrally above the back sideWb of the wafer W.

The water-soluble resin supply source 47 next supplies the water-solubleresin to the nozzle 45, and the water-soluble resin is dropped in apredetermined amount onto the back side Wb of the wafer W. The holdingtable 40 is rotated at a low speed, whereby the dropped water-solubleresin flows from the center of the back side Wb toward its outercircumference under a centrifugal force and spreads over the entiresurface of the back side Wb. As a consequence, a water-soluble resinlayer J is formed with a substantially uniform thickness as illustratedin FIGS. 6 and 7. Subsequently, the rotation is continued to spin-drythe water-soluble resin layer J. However, the drying of thewater-soluble resin layer J may also be performed by heating thewater-soluble resin layer J with a heater or xenon flash lamps disposedabove the holding table 40.

(3-2) Formation of Mask in Mask Forming Step

The wafer W with the water-soluble resin layer J formed thereon istransferred to the laser processing apparatus 1 depicted in FIG. 8, andis held on the holding surface 10 a of the chuck table 10 under suctionvia the tape T. The laser processing apparatus 1 includes, on acircumference of the chuck table 10, fixing clamps 103 evenly atpredetermined intervals in a circumferential direction to fix theannular frame F, and therefore the annular frame F is clamped and fixedby the fixing clamps 103.

The position of desired one of the streets S, the desired street S beingto serve as a reference for irradiating the laser beam onto the wafer W,is then detected by the alignment means 15. Described specifically, animage of the streets S on the front side Wa is captured, for example, bythe image pickup means 150 (for example, an infrared camera) by causinglight, for example, an infrared ray to transmit from the back side Wb ofthe wafer W, and on the basis of the captured image so formed, thealignment means 15 performs image processing such as pattern matching sothat a coordinate position of the desired one street S on the wafer Ware detected.

Concomitantly with the detection of the desired one street S, the chucktable 10 is moved in the direction of the Y-axis under control by theunillustrated control means to perform an alignment between the street Sand the condenser 111 of the laser beam irradiation means 11. Theposition of a focal point of a laser beam to be focused by the condenserlens 111 a is then aligned with the height position of the water-solubleresin layer J. The laser oscillator 119 oscillates a laser beam of awavelength having absorption in the water-soluble resin layer J, and thelaser beam is focused and irradiated onto the water-soluble resin layerJ. In addition, the wafer W is fed at a predetermined processing feedrate in the direction of −X (to the side below the plane of the drawingsheet), that is, in the direction out of the drawing sheet, andtherefore the laser beam is progressively irradiated onto thewater-soluble resin layer J along the street S, whereby thewater-soluble resin layer J is removed through fusion and evaporationand the back side Wb of the substrate W1 is exposed corresponding to thestreet S.

When the wafer W has advanced in the direction of −X to a predeterminedposition where the irradiation of the laser beam along the street S isto be ended, the irradiation of the laser beam is stopped, and at thesame time the chuck table 10 is moved in the direction of the Y-axis toperform an alignment in the direction of the Y-axis between the streetS, which is adjacent to the street S served as the reference in theprocessing feed in the direction of −X, and the condenser 111. The waferW is then fed for processing in the direction of +X (to the side abovethe plane of the drawing sheet), that is, in the direction into thedrawing sheet. Similar to the irradiation of the laser beam in thedirection out of the drawing sheet, the water-soluble resin layer J isremoved and the back side Wb of the substrate W1 is exposedcorresponding to the street S. After similar laser beam irradiation hasbeen performed sequentially along all the streets S extending in thedirection of the X-axis, the chuck table 10 is rotated 90 degrees and asimilar laser beam irradiation is performed. As a consequence, a mask J1depicted in FIG. 9 is formed on the back side Wb of the substrate W1 atregions other than those corresponding to the streets S.

The mask forming step is, however, not limited to the above-describedexample. For example, the mask J1 may also be formed with a resistinstead of the water-soluble resin. Further, a mask may also be formedby bonding a die attach film (DAF), which is in the form of a circularsheet having a diameter substantially equal to or greater than the waferW, to the back side Wb of the wafer W and then irradiating the laserbeam onto the DAF along the streets S from the back side Wb to exposethe substrate W1 corresponding to the streets S. Using liquid DAFinstead of the sheet-shaped DAF, a mask of DAF may be formed insubstantially similar procedures as the formation of the mask J1 of thewater-soluble resin. As a still further alternative example, a mask mayalso be formed by photolithography on the back side Wb of the wafer W.

(46) Plasma Etching Step

After the formation of the mask J1, etched grooves are formed along thestreets S by applying plasma etching to the substrate W1 of the wafer Wthrough the mask J1, for example, with a plasma etching apparatus 9illustrated in FIG. 10.

The plasma etching apparatus 9 illustrated in FIG. 10 includes holdingmeans 90 configured to hold the wafer W, a gas blow head 91 configuredto blow gas, and a chamber 92 with the holding means 90 and gas blowhead 91 internally accommodated therein. The plasma etching apparatusmay also be an inductively coupled plasma etching apparatus that appliesplasma-generating high-frequency power to an induction coil to activateetching gas into a plasma through interaction with a magnetic fieldproduced by the induction coil.

For example, the holding means 90 is an electrostatic chuck, is formedof a dielectric such as a ceramic, and is supported from below by asupport member 900. Inside the holding means 90, a disk-shaped electrode901, which generates electric charges upon application of a voltage, isdisposed in parallel to a holding surface 90 a of the holding means 90,and the electrode 901 is connected to a matching device 94 a and a biashigh-frequency power supply 95 a.

Inside the holding means 90, for example, an unillustrated water flowchannel is formed. With cooling water circulating through the water flowchannel, the holding means 90 is internally cooled to a predeterminedtemperature. Heat transfer gas such as He gas is configured to flow at apredetermined pressure between the holding surface 90 a and the wafer Wheld on the holding surface 90 a so that the efficiency of heatabsorption by the cooling water from the wafer W can be improved. Theholding means 90, for example, is not limited to the single-electrodeelectrostatic chuck exemplified in the figure, but may also be adual-electrode electrostatic chuck.

Inside the gas blow head 91 disposed movably up and down via a bearing919 in an upper part of the chamber 92, a gas diffusion space 910 isarranged. A gas inlet port 911 communicates to an upper part of the gasdiffusion space 910, and a plurality of gas delivery holes 912communicates to a lower part of the gas diffusion space 910. Theindividual gas delivery holes 912 open at lower ends thereof toward theholding surface 90 a of the holding means 90. In a gas supply section 93connected to the gas inlet port 911, fluorine-based gas such as, forexample, SF₆, CF₄, C₂F₆ or C₄F₈ is stored as etching gas.

To the gas blow head 91, a high-frequency power supply 95 is connectedvia a matching device 94. High-frequency power is supplied from thehigh-frequency power supply 95 to the gas blow head 91 via the matchingdevice 94, whereby the etching gas delivered from the gas delivery holes912 can be activated into a plasma.

The plasma etching apparatus 9 includes an unillustrated controlsection, and conditions such as the delivery rate and time of the gasand the high-frequency power are controlled by the control section.

An exhaust port 96 is formed through a bottom of the chamber 92, and anexhaust system 97 is connected to the exhaust port 96. By actuating theexhaust system 97, the interior of the chamber 92 can be depressurizedinto a vacuum atmosphere. Further, the chamber 92 is provided at a sidewall thereof with a loading/unloading port 920 and a gate valve 921configured to open and close the loading/unloading port 920.

Inside the chamber 92, an anti-overheating frame guard 98 is disposed toprevent overheating of the annular frame F during plasma etching. Theanti-overheating frame guard 98 is, for example, one obtained by formingstainless steel (SUS) or the like, which has durability to etching gas,into an annular plate, and is disposed on an inner side wall of thechamber 92 so that the anti-overheating frame guard 98 extends inward ina radial direction.

The formation of the etched grooves in the substrate W1 in thisembodiment may preferably be performed, for example, by a Bosch processthat alternately repeats plasma etching with SF₆ gas and deposition of aprotective film on side walls or the like of grooves with C₄F₈ gas. Asan alternative, the etched grooves may also be formed in the substrateW1 by plasma etching with SF₆ gas alone.

In this step, the wafer W is first loaded into the chamber 92 from theloading/unloading port 920, and is mounted on the holding surface 90 aof the holding means 90 with the side of the mask J1 directed upward.The gate valve 921 is then closed, and the interior of the chamber 92 isbrought into a vacuum atmosphere by the exhaust system 97. The annularframe F from which the wafer W is supported is upwardly covered by theanti-overheating frame guard 98.

The gas blow head 91 is lowered to a predetermined height position. SF₆gas is supplied from the gas supply section 93 into the gas diffusionspace 910, and is blown downward from the gas delivery holes 912. Inaddition, high-frequency power is applied from the high-frequency powersupply 95 to the gas blow head 91, and a high-frequency electric fieldis generated between the gas blow head 91 and the holding means 90 toactivate the SF₆ gas into a plasma. In parallel with this, a voltage isapplied from the bias high-frequency power supply 95 a to the electrode901 to allow the dielectric polarization phenomenon to occur between theholding surface 90 a of the holding means 90 and the wafer W, wherebythe wafer W is attracted and held on the holding surface 90 a via thetape T by an electrostatic attraction force produced by polarization ofelectric charges.

The SF₆ gas, which has been activated into the plasma, progressivelyperforms isotropic etching of the substrate W1 in its regionscorresponding to the streets S without any substantial etching of theback side Wb of the wafer W at the regions where the mask J1 is formed.Thermal effects of the SF₆ gas, which has been activated into theplasma, on the annular frame F are suppressed by the anti-overheatingframe guard 98 that covers upwardly of the annular frame F.

Next, the C₄F₈ gas is supplied from the gas supply section 93 into thegas diffusion space 910, and is blown downward from the gas deliveryholes 912. The high-frequency power is applied from the high-frequencypower supply 95 to the gas blow head 91, and in addition thehigh-frequency power is applied from the bias high-frequency powersupply 95 a to the electrode 901 to activate the C₄F₈ gas into a plasma,whereby fluorocarbon films are allowed to deposit as protective films onthe side walls and bottoms of the etched grooves formed by isotropicetching. Again, SF₃ gas is supplied into the chamber 92 and is activatedinto a plasma to perform anisotropic etching so that only the protectivefilms on the bottoms of the etched grooves are removed. Then, isotropicetching of the substrate W1 exposed in the bottoms of the etched groovesis performed again. Taking the above-described isotropic etching,protective film deposition and anisotropic etching as a single cycle,this cycle is performed, for example, several tens of times, wherebyvertical deep etching of the substrate W1 is realized at a high speedwith a desired aspect ratio to form grid-patterned etched grooves Me inthe substrate W1 along the streets S as depicted in FIG. 11.

Fluorine-based etching gas does not etch the device layer D1 formed froma metal or the like. The plasma etching is, therefore, ended afterperforming it until the upper surface of the device layer D1 is exposedin the bottom of each etched groove Me without the bottom of the etchedgroove Me reaching the device layer D1. Described specifically, theintroduction of the etching gas or the like into the chamber 92 and thesupply of the high-frequency power to the gas blow head 91 are stopped,and the etching gas in the chamber 92 is exhausted to the exhaust system97, thereby creating, inside the chamber 92, a state that no etching gasexists. As a consequence, as depicted in FIG. 11, the etched grooves Me(fully cut grooves) are formed in the substrate W1 along the streets S,and the regions C surrounded by the etched grooves Me are defined in thesubstrate W1.

As an alternative, plasma etching may be performed until the wafer W isbrought into a state that the substrate W1 remains as an etch leftoverwith a small thickness in the bottom of each etched groove Me. In thiscase, the etch leftover may preferably have a thickness of 10 μm orsmaller.

According to the Bosch process, the aspect ratio of each etched grooveMe can be controlled by changing parameters when causing SF₆ gas andC₄F₈ gas to alternately flow into the chamber 92. Each etched groove Mecan, therefore, be formed so that the positions of its opposite wallssubstantially align with the respective positions of the correspondingtwo guide grooves M.

As described above, in the wafer processing method according to thepresent invention, plasma etching is applied through the mask J1 fromthe back side Wb of the wafer W rather than the front side Wa of thewafer W to form a plurality of the etched grooves Me in the substrate W1along the streets S, thereby avoiding the occurrence of a situation suchthat during the etching, the front side Wa of the wafer W is exposed atareas where the mask is thin, and the corresponding devices D aredamaged.

If the mask J1 is not one formed from a water-soluble resin (forexample, is a resist film), the mask J1 may be removed from thesubstrate W1 through ashing or the like by the plasma etching apparatus9 before or after a water immersion step to be performed next.

(5) Water Immersion Step

The wafer W with the etched grooves Me formed therein as depicted inFIG. 11 is next transferred into a bottomed cylindrical water bath 5depicted in FIG. 12. The water bath 5 enables immersion of the wafer W,which is supported by the annular frame F, in its entirety, isconfigured of a side wall 51 and a bottom plate 50 integrally connectedto a lower part of the side wall 51, and stores water (for example, purewater) therein. On an upper surface of the bottom plate 50, anultrasonic vibrator 53 is disposed. The ultrasonic vibrator 53 isformed, for example, in a disk shape by a plurality of piezoelectricelements arranged side by side. To the ultrasonic vibrator 53,unillustrated terminals are connected. A high-frequency power supply 55is connected to the ultrasonic vibrator 53 via the terminals and wires54 to apply an alternate current voltage across the ultrasonic vibrator53. The shape, the disposed location and the like of the ultrasonicvibrator 53 are not limited to this embodiment.

At an area of the upper surface of the bottom plate 50, the area beingon a side outer than an area where the ultrasonic vibrator 53 isdisposed, for example, frame rests 52 are disposed evenly atpredetermined intervals in a circumferential direction to mount theannular frame F thereon. As an alternative, the frame rests 52 may beintegrally formed in an annular shape, or may be provided with clamps toclamp and fix the annular frame F.

The water bath 5 may preferably be configured including the frame rests52 as in this embodiment, but the water bath 5 may be configured withoutincluding the frame rests 52. Described specifically, the annular frameF is thicker than the wafer W so that the annular frame F, which hasbeen transferred into the water bath 5 and supports the wafer W thereon,may be directly mounted on the bottom plate 50. It is, however, notpreferred if the wafer W sags in the water bath 5 and comes into contactwith the ultrasonic vibrator 53. It is, hence, preferred as in thisembodiment to arrange the frame rests 52 to form a certain degree ofclearance between the wafer W and the ultrasonic vibrator 53.

In the water immersion step in this embodiment, as illustrated in FIG.13, for example, the wafer W is immersed in a direction such that theultrasonic vibrator 53 and the back side Wb of the wafer W face eachother and the annular frame F is mounted on upper surfaces of the framerests 52. If the ultrasonic vibrator 53 is disposed at a height abovethe wafer W immersed in the water bath 5, the wafer W may be immersed inthe water bath 5 with the back side Wb directed upward.

High-frequency power is supplied, for example, at an output of 200 Wfrom the high-frequency power supply 55 to the ultrasonic vibrator 53,and the ultrasonic vibrator 53 converts the high-frequency power tomechanical vibrations primarily in an up-and-down direction, whereby theultrasonic vibrator 53 generates ultrasonic vibrations at apredetermined frequency (for example, 25 kHz). The generated ultrasonicvibrations then propagate to the wafer W via water, the wafer W vibratesto produce a shear stress in a vertical direction in the device layer D1at locations corresponding to the etched grooves Me, and cracks areformed in the device layer D1 along the outer peripheral edges of thechip regions C or the device layer D1 is ruptured along the outerperipheral edges of the chip regions C. The output of the high-frequencypower supply 55 may preferably be 200 to 1,000 W, while the frequency ofthe ultrasonic vibrations may preferably be 25 to 120 kHz.

In the water immersion step in this embodiment, the wafer W is immersedin the direction such that the ultrasonic vibrator 53 and the back sideWb of the wafer W face each other. Therefore, cracks occur more readilyin the device layer D1 of the wafer W along the outer peripheral edgesof the chip regions C or the device layer D1 ruptures more readily alongthe outer peripheral edges of the chip regions C. One of reasons for theavailability of such advantageous effects can be attributed to thedirect application of shock waves to the device layer D1 when cavitationbubbles generated in water under ultrasonic vibrations rise in theetched grooves Me of the wafer W, come to contact with the device layerD1, and hence collapse there.

Further, the guide groove forming step is first performed to form theguide grooves M in the device layer D1 in this embodiment, so that withthe formed guide grooves M serving as starting points, cracks are formedin the device layer D1 or the device layer D1 is ruptured. It is,therefore, possible to avoid damage to the devices D and also to preventthe device layer D1 from having dimensions greater than the substrate W1of the chips (protruding parts of the device layer D1 from having largerdimensions). Especially in this embodiment, two guide grooves M areformed in the device layer D1 as rupture starting points correspondingto each street S so that the two guide grooves M substantially alignwith the opposite walls of the corresponding etched groove Me, andtherefore protruding parts of the device layer D1 are minimized.

The mask J1 on the chip regions C may be removed, for example, in thewater bath 5, or may be cleaned off, for example, in a cleaningapparatus different from the water bath 5. If the mask J1 on the chipregions C is removed in the water bath 5, the mask J1 made from thewater-soluble resin is dissolved and removed by ultrasonic cleaning. Thewater bath 5 is provided, for example, with an unillustrated drainagesystem and water supply system. Spent water in the water bath 5, inwhich the mask J1 has dissolved, is drained through the drainage system,and fresh water is additionally supplied from the water supply systeminto the water bath 5.

(6) Separation of Devices from Tape

The wafer W, in which cracks have occurred in the device layer D1 alongthe outer peripheral edges of the chip regions C surrounded by theetched grooves Me or the cracks have occurred along the outer peripheraledges of the chip regions C, is raised from the water bath 5, subjectedto natural drying, blow drying, spin drying or the like, and thentransferred, for example, to a pickup apparatus 6 illustrated in FIG.14. If a UV-curable glue is used in the glue layer Tc of the tape T, anultraviolet ray may be irradiated onto the tape T before performing theseparation of the devices D from the tape T in the pickup apparatus 6,whereby the glue layer Tc is cured to have a reduced adhesive force.

In the pickup apparatus 6, the annular frame F is fixed by unillustratedclamps or the like, and each chip region C is picked up with its uppersurface, from which the mask J1 has been removed, being held undersuction by a suction pad 61. As a consequence, the corresponding deviceD is separated from the tape T. Here, at parts of the device layer D1where cracks have occurred along the outer peripheral edges of the chipregion C, the device layer D1 is ruptured along the outer peripheraledges of the chip region C by the pick-up. Upon the separation, eachdevice D may be pushed upward from a lower side through the tape T, forexample, by an unillustrated needle configured to be movable up and downin the direction of the Z-axis.

As a consequence, a device chip C1 can be formed with the device D anddevice layer D1 included therein as illustrated in FIG. 14.

If the mask J1 is formed from a DAF, for example, the mask J1 is notremoved from each chip region C in the water immersion step, but the DAFon each chip region C is held under suction and picked up by the suctionpad 61 so that the corresponding device D is separated from the tape T.As a consequence, it is possible to form the device chip C1 that can bepacked or stacked on another substrate via the DAF and has the device Dand device layer D1.

The separation of the devices D from the tape T is not limited topick-up that as described above, forms the device chips C1. For example,the individual devices D may be separated all together from the tape Tby bonding a circular tape, which is different from the tape Tillustrated in FIG. 14, to the entire back side Wb of the wafer W andtransferring the wafer W from the tape T to the different tape.

As has been described above, the wafer processing method according tothe present invention can obtain the device chips C1 by separating thedevices D from the tape T (for example, by picking up the chip regionsC) after the performance of the water immersion step. Further, there isno potential risk that the device chips C1 could be reduced in flexuralstrength, because no irradiation of a laser beam of high power isperformed onto the substrate W1 of the wafer W in any of the steps inthe processing method according to the present invention.

The individual steps in the wafer processing method according to thepresent invention are not limited to the above-described embodiment, andobviously can be performed in various different modes within the scopeof the technical concept of the present invention. Concerning elementssuch as the laser processing apparatus 1 and the plasma etchingapparatus 9 depicted or illustrated in the accompanying drawings, theyare not limited to such exemplified configurations either andmodifications are feasible as needed insofar as the advantageous effectsof the present invention can be brought about.

The guide groove forming step can be performed at any stage before theperformance of at least the water immersion step and the tape bondingstep. However, its performance as a first step in the processing methodas in this embodiment is preferred because upon sequentially practicingthe subsequent individual steps, the number of upside-down operations ofthe front side Wa and back side Wb of the wafer W can be decreased.

The guide groove forming step is not an essential step, becausedepending on the thickness and configuration of the device layer D1, therupture or the like of the device layer D1 along the etched grooves Meis possible, without the guide grooves formed in the device layer D1,upon the application of ultrasonic vibrations in the water immersionstep. Especially if a workpiece is a wafer with unillustrated guardrings arranged along the outer peripheral edges of the devices D or in alike case, the need for practicing the guide groove forming step islowered further because, although the omission of guide grooves in thedevice layer D1 may result in incomplete division of the device layer D1even when ultrasonic vibrations are applied in the water immersion step,the device layer D1 can be completely divided when the chips areindividually picked up in the separation of the devices D from the tapeT. In this case, the device layer D1 is torn off so that depending onthe kind of the device layer D1, the device layer D1 may separate beyondthe streets S. However, the arrangement of guard rings along the outerperipheral edges of the devices D prevents separation of the devicelayer D1 beyond the streets S.

The tape bonding step can be performed at any stage before theperformance of the water immersion step. It is, however, preferred toperform the tape bonding step before the performance of the mask formingstep and plasma etching step as in this embodiment, because in the maskforming step and plasma etching step, handling of the wafer W can bereadily performed via the annular frame F.

The present invention is not limited to the details of theabove-described preferred embodiment. The scope of the invention isdefined by the appended claims and all changes and modifications as fallwithin the equivalence of the scope of the claims are therefore to beembraced by the invention.

What is claimed is:
 1. A wafer processing method including a substrateand a device layer stacked over a surface of the substrate, the waferhaving devices formed in respective regions divided by a plurality ofintersecting streets, the method comprising: a mask forming step offorming, on a back side of the wafer, a mask to be used in forming aplurality of etched grooves in the substrate along the streets from theback side of the wafer; a plasma etching step of applying plasma etchingfrom the back side of the wafer through the mask after performing themask forming step, thereby forming the etched grooves in the substratealong the streets and defining chip regions surrounded by the etchedgrooves; a water immersion step of immersing the wafer in water, towhich ultrasonic vibrations are being applied by an ultrasonic vibrator,after performing the plasma etching step, thereby forming cracks in thedevice layer along outer peripheral edges of the chip regions orrupturing the device layer along the outer peripheral edges of the chipregions; and a tape bonding step of bonding a tape to a front side ofthe wafer before performing at least the water immersion step, whereinthe devices are separated from the tape after performing the waterimmersion step.
 2. The wafer processing method according to claim 1,further comprising: before performing at least the water immersion stepand the tape bonding step, a guide groove forming step of forming guidegrooves in the device layer along the streets from the front side of thewafer by a cutting blade or a laser beam without allowing the guidegrooves to reach the substrate.
 3. The wafer processing method accordingto claim 1, wherein in the water immersion step, the wafer is immersedin a direction such that the ultrasonic vibrator and the back side ofthe wafer face each other.